1. Field of the Invention
This invention relates to a radiation image read-out apparatus for exposing a stimulable phosphor sheet, on which a radiation image has been stored, to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, photoelectrically detecting the emitted light, and thereby reading out the radiation image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
In general, in radiation image recording and reproducing systems which use stimulable phosphor sheets, radiation images are read out with a read-out apparatus described below. Specifically, a beam of stimulating rays is deflected by a light deflector and caused to scan a stimulable phosphor sheet in a main scanning direction. Also, the stimulable phosphor sheet is moved in a subscanning direction, which is approximately normal to the main scanning direction.
As the light deflector, a rotating polygon mirror may be utilized. The rotating polygon mirror can carry out scanning more reliably than other types of light deflectors, such as galvanometer mirrors. However, for this purpose, the rotating polygon mirror must be rotated quickly. On the other hand, in order for a stimulable phosphor sheet to be stimulated, it must be exposed to stimulating rays having a comparatively high energy. However, when the rotating polygon mirror is rotated quickly, the speed, with which the stimulating ray beam scans the stimulable phosphor sheet in the main scanning direction, becomes high, so that the level of stimulation energy which the stimulable phosphor sheet receives becomes low. As a result, the intensity of light emitted by the stimulable phosphor sheet becomes low, and therefore the signal-to-noise ratio (S/N ratio) of an image signal thus obtained cannot be kept high.
In view of the above circumstances, in U.S. Pat. No. 4,748,326, the applicant proposed a radiation image read-out apparatus with which a read-out image signal having a high S/N ratio can be obtained even when a stimulable phosphor sheet is quickly scanned with a stimulating ray beam.
With the proposed radiation image read-out apparatus, a stimulable phosphor sheet, on which a radiation image has been stored, is scanned with a stimulating ray beam, which causes the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and the emitted light is photoelectrically detected. The beam diameter of the stimulating ray beam is adjusted such that it is smaller than the size of each picture element in the radiation image. Also, each picture element is divided into a plurality of portions, and each portion is scanned with the stimulating ray beam by a single scanning step. A plurality of image signal components, which are obtained during a plurality of scanning steps and represent a single picture element, are added by an addition means. An image signal component resulting from the addition is taken as a read-out image signal component representing a single picture element.
When the beam diameter of the stimulating ray beam is adjusted such that it is sufficiently small, a high level of stimulation energy can be obtained. Also, when different portions of a single picture element are scanned by different scanning steps, the amount of light emitted by a stimulable phosphor sheet per picture element can be kept sufficiently large. The read-out image signal component, which is obtained by adding a plurality of image signal components representing a plurality of portions of a single picture element, represents the total amount of light emitted by the stimulable phosphor sheet per picture element. Therefore, the read-out image signal component thus obtained has a high S/N ratio.
However, study carried out by the inventor revealed that the radiation image read-out apparatus proposed in U.S. Pat. No. 4,748,326 has the drawback in that the efficiency, with which stimulating rays are utilized, becomes low.